The free radical initiated polymerization of vinyl monomers in aqueous emulsions is a process well known in the polymer field and described in such classic references as "Emulsion Polymerization" by F. A. Bovey, et al., Interscience Publishers, N.Y., 1955 and in the Encyclopedia of Polymer Science and Technology. The vinyl latices are prepared by suspending one or more monomers in water in contact with a surfactant and a free radical initiator such as ammonium persulfate. Surfactants commonly employed include ethoxylated polypropylene glycol, ethoxylated nonyl phenol, etc. Mixtures of various surfactants can be employed including the ammonium or alkali salts of low molecular weight polymers containing carboxy groups. The use of such low molecular weight carboxylated polymers as cosurfactant typically leads to the characterization of the resultant polymer vinyl latex as a fortified or resin-fortified latex.
Artisans engaged in the development of inks and surface coatings derived from polymer latices are continually challenged by the need to modify the latex formulations to enhance properties required for the end-user application. Of these modifications, the inclusion of wax in the latex formulation is commonly employed and is well known in the art. Wax, it has been found, functions well as a component of a vinyl latex to provide key properties in the coating produced therefrom. As will be described in more detail hereinafter, a wide variety of waxes can be and have been included in vinyl latex. These waxes, generally added in low concentrations to the vinyl latex, provide coatings with improved mar resistance, anti-blocking properties, slip and formability improvement, anti-settling, coating flatting, abrasion resistance and metal marking resistance.
As practiced in the art heretofore, waxes are added to the latex polymerization reaction mixture just before, during, or after the emulsification polymerization step. A major problem found within the latices prepared by this technique is a propensity for the wax to separate from the emulsion mixture, or otherwise cause the emulsion to break. Separation can occur as a function of the work preformed during application, during post-treatment of the coating formed in the application, as a result of ambient temperature swings or progressive chemical incompatibility. For whatever cause, the emulsion instability and resultant wax separation destroys the utility of the latex product. The problem of emulsion stability becomes particularly aggravated when higher concentrations of wax are required in the emulsion in order to confer a particularly desirable property on the resultant coating. The higher the wax loading, the more difficult it is to prepare a stable emulsion.
One of the more desirable coating properties sought after in the field of inks and synthetic surface coatings is moisture impermeability. It is highly advantageous for coatings such as those used on food and detergent containers to be very resistant to, if not impermeable to, moisture. One way to produce such moisture impermeability in vinyl latex derived coatings is to include relatively high loadings or concentrations of wax in the latex. However, a wax concentration high enough to render the subsequent coating a moisture barrier intrudes into the zone of those latex/wax mixtures that comprise highly unstable emulsions. Accordingly, workers in the field are critically challenged by the need to discover a means to prepare vinyl latices that are both stable as formulated but also contain high concentrations of wax so that coatings can be produced having improved moisture barrier qualities.
U.S. Pat. No. 4,151,143 describes a two stage process for the preparation of fortified, surfactant-free polymer emulsion. The first stage comprises the polymerization of a mixture of monomers including carboxylic containing monomers followed by neutralization of the polymer product. In a second stage a mixture of monomers including acrylate monomers and a polymerization catalyst are added to the emulsion produced in the first stage. No cosurfactant is utilized and the process does not teach the incorporation of high concentrations of wax.
U.S. Pat. No. 4,820,762 describes the preparation of a fortified latex composed of a preformed soluble resin, a cosurfactant and latex forming monomers. The soluble resin is dispersed in water or alkali and comprises a resin having a low molecular weight. The process does not teach the incorporation of high wax concentration in a fortified latex.
U.S. Pat. No. 4,293,471 teaches the production of a fortified emulsion polymer by first preparing an aqueous dispersion of an alkyd resin neutralized to a pH of about and then forming an emulsion polymer from one or more vinyl monomers in aqueous dispersion. The patent does not teach the production of high wax content emulsions.
U.S. Pat. No. 4,569,896 teaches the production of a toner composition which includes resin particles of styrene methacrylate copolymer grafted or containing a low molecular weight wax plus a second resin composed of a terpolymer of styrene, acrylate and acrylonitrile. The developer also contains magnetite particles and carbon black.
Japanese Patent JP 59,191,706 (CA vol. 102:62720t) teaches styrene grafted polyolefin waxes useful as release agents for molded plastics. The polymers are prepared by melt polymerization. The patent does not teach the preparation of fortified emulsions for moisture barrier coatings.
It is an object of the present invention to provide a process for the production of a stable vinyl latex that contains a high concentration of wax and is suitable for the formation of coatings that exhibit superior moisture barrier properties.
Another object of the present invention is the preparation of high wax content aqueous emulsions by the emulsion polymerization of vinyl monomers in the presence of a soluble resin cosurfactant containing said wax.
Yet another object of the present invention is the production of a high wax content soluble resin useful as a cosurfactant in aqueous emulsion polymerization wherein the soluble resin is prepared in the presence of wax under graft polymerization conditions.